This invention relates to a solenoid-operated fluid control valve for providing steering assist in the power steering system of a motor vehicle and more particularly to an electrical control valve which varies fluid hydraulic flow through the valve in relation to vehicle speed to effect a reduction in the level of assist provided in response to high speed vehicle operation.
Steering systems in which the hydraulic flow is controlled in relation to vehicle speed so as to effect a reduction in the level of assist provided during vehicle operation are known. Illustrative of one such system is "Flow Regulated Power Assist Steering System with Evasive Maneuver Capability" U.S. Pat. No. 4,629,025, issued Dec. 16, 1986. In this system, a solenoid-operated flow control valve is located in the fluid path of the power cylinder of the steering unit and is controlled by an electronic control unit (ECU) in response to vehicle speed and handwheel position signals. Generally, fluid flow is reduced as vehicle speed increases, thus providing a high level of hydraulic flow (and steering assist) at low vehicle speeds, such as in parking lot maneuvers and fast wheel movement by the user, and a lower level of hydraulic flow (and steering assist) at highway speeds to improve on-center stability of the vehicle.
The above-disclosed flow control valve has a complex arrangement of fluid chambers, flow paths and plungers which move to open or close a fixed orifice whereby to control fluid flow. A simpler and more cost-effective solenoid control valve that modulates steering effort, according to vehicle speed, is desirable.
It has long been an object in the design of solenoid valves to improve mechanical reliability and reduce the costs of manufacturing and assembly costs. Although a multi-part assembly is costly to manufacture, a solenoid valve must be assembled from several parts with the operation of the valve requiring that preselected of the parts be comprised only of a magnetizable material whereby to function as a part of the requisite magnetic circuit and other of the parts be nonmagnetic.
In an effort to reduce the number of parts needed without affecting the requisite magnetic circuit, several parts of magnetic and nonmagnetic material have been brazed together to form a composite structure for supporting the movable armature of the valve. However, the brazing operation requires close control of temperature versus time in the process furnace, which is difficult to control in high production, and if not brazed enough results in waste and high rejection rate, thereby increasing the costs of manufacture. Additionally, a bimetallic support assembly is prone to the effects of different thermal expansion rates which inhibits its use in applications subject to temperature differentials.
In accordance with the invention herein, a solenoid operated fluid control valve, which is adapted to be mounted in a fluid passage to control fluid flow therethrough, includes a protective dielectric housing molded about a solenoid coil, a magnetically responsive housing comprising a magnetizable fluid fitting (i.e., the pole piece) provided with a closable orifice and fluid inlet and outlet passages, and a pair of sleeves of magnetizable material assembled to the dielectric housing; a one-piece, generally cylindrical cup-shaped core tube comprised of a nonmagnetizable material connected to one end of the fitting whereby to form a subassembly that is removably inserted into the coil; and an armature assembly movably mounted in the core tube and including a nonmagnetic plunger to restrict fluid flow through the orifice and a magnetizable armature. A nonmagnetic ring separates the pole piece from the armature to form an air gap therebetween and to space the plunger end from closing relation with the fluid inlet.
A magnetic circuit is formed by the concentric sleeves, the fluid fitting, and the armature. As a result of electrical current being provided to the coil, a magnetic field is induced in the magnetic circuit and magnetic flux passed therethrough whereby the plunger is moved relative to the fluid inlet.
The core tube is of unitary construction, defines a fluid pressure chamber sized to receive and position the armature relative to the coil, and is configured to assist in the efficient assembly of the magnetic portions of the valve relative to the electrical coil. The cylindrical wall of the core tube supports the inner sleeve and forms a balanced radial magnetic gap around the armature whereby to obviate pull forces on the armature. The strength of the magnetic field is enhanced by radially directing the magnetic flux between the outer and inner sleeves, first by conically shaping the confronting end faces of the armature and pole piece for nested fitment and separated by a like-shaped air gap, second by abutting one end of the outer sleeve with a pole piece flange, and third by abutting a radially outwardly extending flange on the inner sleeve against the other end of the outer sleeve.
The core tube herein advantageously provides an armature structure which is less expensive to make and enhances the rapid and cost efficient assembly of necessary valve parts, provides a closely toleranced part, is durable and rugged, and formed by a cost-effective process in which accuracy is easy to control.
Additionally, the one-piece core tube eliminates joints which could have an adverse effect on the concentricity needed for precise and rapid positioning and repositioning of the armature to achieve optional flow control, obviates the problems associated with metal joining operations, and enhances its use in fluid pressure systems requiring the armature housing to function as a fluid pressure containment device.
A retention ring is easily connected to one end of the core tube and against the inner sleeve to assist in the parts being retained in operable relation to the dielectric housing, the ring being removably mounted for disassembly and repair of the valve.
Additional objects, advantages and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.